Alkaline storage batteries are less susceptible to environment and charging/discharging conditions, and have thus been recently receiving attention as power sources for hybrid electric vehicles (hereinafter, abbreviated as HEV) while yielding demand to lithium ion secondary batteries as power sources for portable devices.
HEV power sources are more often discharged in a pulsed mode rather than in a continuous mode, unlike power sources for portable devices. Therefore, the system efficiency has been improved by enhancing the output characteristic per cell, rather than by simply increasing the number of cells to be connected in series. In view of this, attempts have been made to lower the direct-current resistances in the HEV power sources in particular by reducing the resistances of the components, improving the reactivity of the active materials, and the like. One of the examples is an improvement to the structure of the positive electrode.
Positive electrodes for alkaline storage batteries are broadly classified into a sintered type and a non-sintered type. A sintered type positive electrode is formed by impregnating a porous nickel sintered substrate having a porosity of 80% or so, which is obtained by sintering nickel powder, with a nickel salt solution such as an aqueous nickel nitrate solution, and then immersing it into an aqueous alkali solution to precipitate nickel hydroxide active material (for example, Patent Document 1). This sintered substrate contains dense nickel skeletons having a pore size of 10 μm or so, and thus has a high collecting capability. Since the fine nickel skeletons have large surface areas, there is the disadvantage that repetition of charging and discharging oxidizes the nickel skeletons to consume an electrolyte, thereby increasing the direct-current resistance of the battery. Then, there has been proposed a process of applying an antioxidant surface treatment to nickel surfaces so that the nickel skeletons become less prone to oxidation (for example, Patent Document 2).
Meanwhile, a non-sintered type positive electrode uses a foamed three-dimensional porous substrate of nickel metal having a porosity of around 95% as a core substrate, which is filled with nickel hydroxide powder as an active material (for example, Patent Document 3). This core substrate has a porosity higher than those of the sintered type positive electrodes, and thus is capable of increased capacity. The weight ratio of the core substrate in this positive electrode is typically as low as around 15% to 25%, in comparison to 45% to 60% in sintered type positive electrodes, and the content of the active material is accordingly higher. Non-sintered type positive electrodes have thick skeletons of nickel and relatively flat skeleton surfaces, with significantly smaller surface areas as compared to sintered type positive electrodes. The nickel skeletons are thus harder to oxidize and can avoid oxidation which entails the consumption of the electrolyte.
Attempts have been made to achieve a high-output alkaline storage battery for HEV, free of nickel skeleton oxidation, by comprising an non-sintered type positive electrode with a core substrate weight ratio equivalent to that of a sintered type positive electrode, through the use of the technique according to the foregoing Patent Document 2, or through the use of the foamed three-dimensional porous substrate according to Patent Document 3.    Patent Document 1: Publication of Japanese Patent No. 3275594    Patent Document 2: Japanese Patent Laid-Open Publication No. Sho 59-96659    Patent Document 3: Japanese Patent Laid-Open Publication No. Hei 11-242958